Neither "roasted" nor "steamed" are completely appropriate for this cooking method.
While there is steam in the vessel, it comes only from the water lost from the meat, not from any additional water, and the meat is heated by the microwaves, not by the steam.
Without keeping a lid on the cooking vessel, the loss of water is too rapid and the cooked meat becomes too dry. Even so, the weight of meat is reduced to about two thirds, due to the loss of water.
In the past, I was roasting meat on a covered grill, where the air enclosed in it was heated by a gas burner through an opening located on one side, on its bottom. With such a covered grill, the air in which the meat was cooked would also contain steam from the water lost by the meat, so the end result was very similar to the microwave cooking that I do now, also preventing the meat from becoming too dry, unlike with roasting on an open grill, while also concentrating the flavor and avoiding its dilution by added water or oil.
It sounds like this is steamed meat, as opposed to roasted. Your cooking time seems to match a quick search for steamed chicken recipes: https://tiffycooks.com/20-minutes-chinese-steamed-chicken/